Water is a common contaminant in hydraulic and lubricating systems. Moisture can enter these systems at several points:

• Through reservoir breather caps in humid air. System fluid absorbs some of this moisture, while some is condensed on the inside surfaces of the reservoir.
• Through worn seals. Besides humid air, coolant and cutting fluid sprays can enter hydraulic systems through worn rod seals as the cylinder moves. In lubricating systems, these water-based fluids can enter through rotating shaft seals on pumps, machine tool spindles and gear boxes.
• Through heat exchangers. Worn and damaged heat exchangers can allow cooling water to leak through seals and ruptured lines into the oil system, and vice versa.
• In new oil. An oil barrel stored outside in a vertical position is likely to collect rainwater around its bung.

With changes in temperature, some of this moisture will be sucked into the barrel. Eventually, this water enters the system fluid when the reservoir is filled.

First published October 2005